And I think GP's idea is that if companies can't send over your identifying data from client side to advertising companies then they are more than happy to send it over from server side.

I see. I was talking about the fact that the cookies are visible to the ad/tracking company.

In my opinion (and I think this is common) ad/tracking companies are third parties and when a company proxy data to one of them it is then by definition residing with a third party ev3n if they don't share it with their other customers.

An advertising company knows I travelled to wikivoyage and spent lots of time reading about Thailand.

They then know that I (same cookie) visited cnn and read about Ebola

They then know I visited a cycle magazine and read about a new bike.

With proxying they wouldn’t be able to link those three visits together (with cookie data)

When I visit a fourth site, it’s a brand new visit - there is no way for the advertising. Company to know I’m the same version who visited the previous sites.